1. Field of the Invention
The present invention relates to transmission rate judgment methods and their apparatuses.
2. Description of the Related Art
Recently, with drastically growing demands for terrestrial mobile communications such as car telephones and portable telephones, great importance is attached to a technology to allow effective utilization of frequencies to secure capacities for as many subscribers as possible with limited frequency bands. As one of multiple access systems for effective utilization of frequencies, the code division multiple access (CDMA) system is attracting attention. The CDMA system is a multiple access system that utilizes a spread spectrum communication technology, capable of achieving the excellent communication quality through its wideband and sharp correlation characteristics using pseudo noise (PN) series, etc. Terrestrial mobile communication systems using the CDMA system are described, for example, in U.S. Pat. No. 4,901,307.
A mobile communication system using the CDMA system can increase its system capacity by introducing variable rate transmission. For example, in variable rate voice CODEC, the data rate changes in units of a 20 msec section, called "frame" in accordance with the voice activity. Transmitting only the amount of information necessary to send the corresponding voice signal in each frame makes it possible to reduce interference to other users and increase the system capacity as well.
On the transmitting side, after voice coding, the system adds a CRC bit to check the frame quality, for example, and performs convolutional coding. The convolutional-coded data are repeated according to the data rate and have frames of the same length irrespective of the data rate. After being subjected to processing such as block interleave, spreading and modulation, the data are transmitted.
Then, a conventional transmission rate judgment apparatus is explained with reference to drawings. FIG. 1 is a block configuration diagram of a conventional transmission rate judgment apparatus.
In FIG. 1, the conventional transmission rate judgment apparatus judges a transmission rate using variable rate decoding section 501. This variable rate decoding section 501 comprises derepeating section 502 that combines or adds repeated symbols in accordance with the rate to be demodulated, Viterbi decoding section 503 that performs Viterbi decoding on the output symbol of derepeating section 502 and outputs decoded data, and CRC checking section 504 that performs a CRC check on the Viterbi-decoded data. It also comprises convolutional coding section 505 that performs convolutional coding on the output of Viterbi decoding section 503 and symbol error number counting section 506 that calculates the number of symbol errors in a frame using the output of derepeating section 502 and the output of convolutional coding means 505. It further comprises rate judgment section 507 that judges the rate based on the CRC check result and the reliability and the number of symbol errors of Viterbi decoding.
The sections from derepeating section 502 to symbol error number counting section 506 can perform processing according to multiple anticipated data rates, for example, full rate, 1/2 rate, 1/4 rate and 1/8 rate. Since the data rate varies from one frame to another, the system performs this processing for all rates or some rates, and estimates and judges the transmission rate.
Then, the rate judgment procedure is explained with reference to FIG. 2 and FIG. 3. FIG. 2 is a flow diagram showing the procedure for decoding a received symbol and judging the rate. As shown in FIG. 2, after performing the respective processing from derepeating section 502 to symbol error number counting section 506 for all anticipated rates (step 601), the system makes a judgment according to the result (step 602). In step 602, this method makes a judgment using the quality information of all rates, that is, the CRC check results, Viterbi decoding reliability and number of symbol errors of all rates.
FIG. 3 is a flow diagram showing the procedure for judging the rate through decoding of each rate and quality judgment. As shown in FIG. 3, the system carries out processing from derepeating section 502 to symbol error number counting section 506 sequentially starting from a specific rate and makes a judgment in the process. In step 701, it carries out processing from derepeating section 502 to symbol error number counting section 506 for the rate with the largest quantity of information (full rate), and in step 702 it makes a judgment based on the quality information of the full rate. If the judgment result is acceptable, the system judges it as a full rate (step 710) and the process ends here. If the judgment result is not acceptable, the system carries out processing from derepeating section 502 to symbol error number counting section 506 for a 1/2 rate which has half the quantity of information of the full rate (step 703) and in step 704 it makes a judgment based on the quality information of the 1/2 rate. Rate judgment is continued in this way repeating the same sequential processing.
However, the conventional transmission rate judgment apparatus above needs to perform decoding for all rates or multiple rates, having the problem of increasing the amount of processing and hence increasing power consumption. For battery-operated apparatuses, for example portable terminals in particular, an increase in power consumption leads to a decrease of conversation time and waiting time.